Silvestrol and silvestrol derivatives refer to a family of antibacterial and antitumor agents (Lucas, D. M. et al (2009) Blood, 113, 4656-4666; Alinari, L. et al (2012) Clin. Cancer Res., 18, 4600-4611; Cencic, R., et al. (2009) PLoS One, 4, e5223; Hwang, B. Y. et al (2004) J. Org. Chem., 69:3350-3358; U.S. Pat. No. 6,710,075). Silvestrol and analogs are potent and selective protein synthesis inhibitors and have been studied for their anti-hyperproliferative properties (Liu, T. et al (2012) Journal of Medicinal Chemistry, 55(20):8859-8878; WO 2015/085221; WO 2013/016658; WO 2004/041812; U.S. Pat. Nos. 8,137,509; 8,404,088; WO 2006/007634; U.S. Pat. No. 7,816,544).
Silvestrol, (CAS:697235-38-4) is named as methyl (1R,2R,3S,3aR,8bS)-6-(((2S,3R,6R)-6-((R)-1,2-dihydroxyethyl)-3-methoxy-1,4-dioxan-2-yl)oxy)-1,8b-dihydroxy-8-methoxy-3a-(4-methoxyphenyl)-3-phenyl-2,3,3a,8b-tetrahydro-1H-cyclopenta[b]benzofuran-2-carboxylate, and has the structure:

Antibody-drug conjugates, also known as ADC or immunoconjugates, are targeted chemotherapeutic molecules, combining the properties of both antibodies and cytotoxic drugs by targeting potent cytotoxic drugs to the antigen-expressing tumor cells, thereby enhancing their anti-tumor activity. Successful antibody-drug conjugate development for a given target antigen depends on optimization of antibody selection, linker stability, cytotoxic drug potency and mode of linker-drug conjugation to the antibody. More particularly, selective antibody-drug conjugates are characterized by at least one or more of the following: (i) an antibody-drug conjugate formation method wherein the antibody retains sufficient specificity to target antigens and wherein the drug efficacy is maintained; (ii) antibody-drug conjugate stability sufficient to limit drug release in the blood and concomitant damage to non-targeted cells; (iii) sufficient cell membrane transport efficiency (endocytosis) to achieve a therapeutic intracellular antibody-drug conjugate concentration; (iv) sufficient intracellular drug release from the antibody-drug conjugate sufficient to achieve a therapeutic drug concentration; and (v) drug cytotoxicity in nanomolar or sub-nanomolar amounts.
Antibody-drug conjugates allow for the targeted delivery of a drug moiety to a tumor, and, in some embodiments intracellular accumulation therein, where systemic administration of unconjugated drugs may result in unacceptable levels of toxicity to normal cells (Polakis P. (2005) Current Opinion in Pharmacology 5:382-387).
Antibody-drug conjugates are targeted chemotherapeutic molecules which combine properties of both antibodies and cytotoxic drugs by targeting potent cytotoxic drugs to antigen-expressing tumor cells (Teicher, B. A. (2009) Current Cancer Drug Targets 9:982-1004), thereby enhancing the therapeutic index by maximizing efficacy and minimizing off-target toxicity (Carter, P. J. and Senter P. D. (2008) The Cancer Jour. 14(3):154-169; Chari, R. V. (2008) Acc. Chem. Res. 41:98-107.
Antibodies have been developed that provide for site-specific conjugation of a drug to the antibody through cysteine substitutions at sites where the engineered cysteines are available for conjugation but do not perturb immunoglobulin folding and assembly or alter antigen binding and effector functions (Junutula, et al., 2008b Nature Biotech., 26(8):925-932; Dornan et al. (2009) Blood 114(13):2721-2729; U.S. Pat. Nos. 7,521,541; 7,723,485; WO2009/052249). These THIOMAB™ antibodies can then be conjugated to cytotoxic drugs through the engineered cysteine thiol groups to obtain THIOMAB™ drug conjugates (TDC) with uniform stoichiometry (e.g., up to 2 drugs per antibody in an antibody that has a single engineered cysteine site). Studies with multiple antibodies against different antigens have shown that TDCs are as efficacious as conventional antibody-drug conjugate in xenograft models and are tolerated at higher doses in relevant preclinical models. THIOMAB™ antibodies have been engineered for drug attachment at different locations of the antibody (e.g., specific amino acid positions (i.e., sites) within the light chain-Fab, heavy chain-Fab and heavy chain-Fc). The in vitro and in vivo stability, efficacy and PK properties of THIOMAB™ antibodies provide a unique advantage over conventional antibody-drug conjugates due to their homogeneity and site-specific conjugation to cytotoxic drugs.
There are still other limitations or challenges to the design, preparation and use of antibody-drug conjugates. For example, some linkers may be labile in the blood stream, thereby releasing unacceptable amounts of the drug prior to internalization in a target cell (Khot, A. et al (2015) Bioanalysis 7(13):1633-1648). Other linkers may provide stability in the bloodstream, but intracellular release effectiveness may be negatively impacted. Linkers that provide for desired intracellular release typically have poor stability in the bloodstream. Alternatively stated, bloodstream stability and intracellular release are typically inversely related. Second, in standard conjugation processes, the amount of drug moiety loaded on the antibody carrier protein (the drug loading), the amount of aggregate that is formed in the conjugation reaction, and the yield of final purified conjugate that can be obtained are interrelated. For example, aggregate formation is generally positively correlated to the number of equivalents of drug moiety and derivatives thereof conjugated to the carrier-antibody. Under high drug loading, formed aggregates must be removed for therapeutic applications. As a result, drug loading-mediated aggregate formation decreases antibody-drug conjugate yield and can render process scale-up difficult. Accordingly, there is a continuing need for improved efficacious antibody-drug conjugates that provide for optimized safety and efficacy.